TeX source:
\begin{array}{rclcl} 100 \,\% &\widehat{=}& 400 \;\text{g} \cr\cr 1 \,\% &\widehat{=}& \dfrac{400}{100} \;\text{g} \cr\cr 4 \,\% &\widehat{=}& \dfrac{4 \cdot 400\;\text{g}}{100} &=& 16 \;\text{g} \end{array}